IaaS - Infrastructure as a Service provides all the equipment for a customer such as networking, data storage, and hardware with use of internet.
It up to the customer to implement their own Operating System (OS), software, deployments, testing, performance metrics, etc. You are basically
renting the hardware and networking infrastructure. All maintence on the infrastructure is owned by the provider.
Examples: Amazon Web Services (AWS), RackSpace, Microsoft Azure
PaaS - Platform as a Service includes IaaS but customer provides only the application and data. The customer does not have to worry about OS updates,
software upgrades, data storage etc. The customer can focus on the creation of the apps.
Examples: Windows Azure, AWS Elastic Beanstalk, Google App Engine
SaaS - Software as a Service is all of the above plus the applications are created and updated by the provider. The customer is the user.